The present invention relates to the saponification (caustic hydrolysis) of ethylene-alkyl acrylate copolymers.
The batch aqueous saponification of ethylene-alkyl acrylate copolymers is known. U.S. Pat. No. 3,970,626, issued Jul. 20, 1976 to Hurst et al., describes a process for saponifying an ethylene-methyl acrylate copolymer using excess NaOH in aqueous solution. However, the reaction requires long residence times, on the order of several hours. Also, if the saponified product is to be used in dry form, this process requires the separation of large amounts of water from the product.
European Patent Application 0 104 316, published Apr. 4, 1984, describes a method for forming ionic copolymer salts from low molecular weight copolymer acids formed from ethylene and an a,.beta.-ethylenically unsaturated carboxylic acid and oxides of Group II metals. The copolymer acid and metal oxide are mixed at a temperature between the melting point of the copolymer acid and 20.degree. C. above the melting point of the copolymer acid. While the metal oxide is added to the copolymer acid in dry form, water can be added to the reaction mixture to accelerate the reaction.
U.S. Pat. No. 4,638,034, issued Jan. 20, 1987 to McClain, discloses the preparation of ethylene-acrylic acid copolymers or salts thereof by first preparing an ethylene-alkyl acrylate copolymer, and saponifying such ethylene-alkyl acrylate copolymer in the molten state, in the absence of solvent or water other than by-product alkanol, with metal hydroxides to form alkanol and a salt, or an ionomer, of ethylene-acrylic acid copolymer.
U.S. Pat. No. 5,003,001, issued Mar. 26, 1991 to Hasenbein et al., discloses ionically crosslinked ethylene copolymers prepared by mixing a carboxyl-containing copolymer with an aqueous metal salt solution in the reaction zone of a twin-screw extruder and transferring the product to a devolatilization zone. While the ethylene copolymers are said to include copolymers of ethylene and an a,.beta.-ethylenically unsaturated comonomer donating carboxyl groups, i.e. comonomers which are converted into carboxyl-containing comonomers during or after the polymerization by hydrolysis and/or pyrolysis, the only copolymer used in the examples is an ethylene-acrylic acid copolymer.
For a commercial operation, it is desirable to have a rapid and continuous process which utilizes conventional equipment, such as extruders and injection pumps. A process has now been discovered which permits the saponification of ethylene-alkyl acrylate copolymers to be carried out to a high conversion using conventional equipment.